This invention relates to a seismic data gathering system and more particularly to a seismic cable for detecting and transmitting seismic reflection signals to a data acquisition unit.
In seismic exploration seismic energy is generated at a shotpoint at or near the surface of the earth for land exploration or in a water layer for marine exploration. The seismic energy is reflected from subsurface interfaces between subsurface layers of the earth, and is received by a spread of seismic energy detectors on the surface of the earth or in a water layer. These seismic energy detectors, commonly termed "geophones" for land exploration and "hydrophones" for marine exploration, are incorporated into a seismic cable which extends along the earth's surface or is towed in a water layer. The outputs of the seismic detectors are applied to a seismic data acquisition or recording system having a separate recording channel for each detector. These recording channels therefore contain information which can be used to represent the characteristics of the subsurface formations from which the seismic reflection signals have been detected by the seismic detector cable.
A seismic detector, such as a geophone, senses vibrations by means of movement of a coil of electrical conductors relative to a fixed permanent magnet in response to such vibrations. The coil traverses the magnetic line of flux of the permanent magnet and produces an electrical signal which is responsive in amplitude and frequency to the sensed vibrations. It is common practice to provide mechanical or electrical damping of the moveable components of the seismic detector. The present invention is particularly useful in providing a constant electrical damping for a plurality of parallel seismic detectors included in a seismic detector cable which is coupled to the input of a seismic data acquisition or recording system.